1. Field of the Invention
This invention relates to an optical element such as a polarization separating element or a polarization anisotropic hologram element. This invention also relates to a method of fabricating an optical element. This invention further relates to an optical information processing apparatus.
2. Description of the Prior Art
Japanese published unexamined patent application 63-314502 discloses a polarization separating element usable in an optical communication system. The polarization separating element of Japanese patent application 63-314502 includes a substrate of lithium niobate, proton-exchanged layers, and dielectric films. The proton-exchanged layers are periodically formed on a surface of the substrate at intervals. The dielectric films are formed on and aligned with the proton-exchanged layers respectively. In the polarization separating element of Japanese patent application 63-314502, lithium-niobate substrate portions covered with the proton-exchanged layers and the dielectric films alternate with lithium-niobate substrate portions uncovered therefrom. In the following description, the substrate portions covered with the proton-exchanged layers and the dielectric films will also be referred to as the covered substrate portions, while the substrate portions uncovered therefrom will also be referred to as the uncovered substrate portions.
The relation or ratio in refractive index between the lithium-niobate substrate and the proton-exchanged layer varies in accordance with which of an ordinary ray of light and an extraordinary ray of light is applied thereto.
The polarization separating element of Japanese patent application 63-314502 operates as follows. In the case of an extraordinary ray of light, the part of the light which has passed through the covered substrate portions is delayed in phase relative to the part of the light which has passed through the uncovered substrate portions. The phase delay is caused by the difference in refractive index between the dielectric film and air, and the difference in refractive index between the proton-exchanged layer and the lithium niobate substrate. As a result of the phase delay, the extraordinary ray is diffracted by the polarization separating element. In the case of an ordinary ray of light, the dielectric films compensate for a phase difference which might be caused between the part of the light which passes through the covered substrate portions and the part of the light which passes through the uncovered substrate portions. Specifically, the difference in refractive index between the dielectric film and air, and the difference in refractive index between the proton-exchanged layer and the lithium-niobate substrate cancel the phase difference. Thus, the ordinary ray is un-diffracted by the polarization separating element.
During the fabrication of the polarization separating element of Japanese patent application 63-314502, the formation of the dielectric films and the alignment between the dielectric films and the proton-exchanged layers necessitate troublesome and laborious steps.